Deformed bars are notoriously difficult to handle by mechanical means not only because of their length and weight, but also because of their extremely wide variations in shapes, dimensions, chemical composition, strength, stiffness, and other related mechanical properties.
These bars need however to be mechanically handled in order to provide continuity of reinforcement in concrete structures. This continuity of reinforcement is best provided by not reducing the properties of the bars themselves at the location of their connection, while still being feasible in a convenient and economical way.
The first method imagined to create such a convenient and economical connection was to make a thread on the bar ends as shown on FIG. 1, thereby enabling to link them to another bar by means of an internally-threaded connector. Such a solution presents the disadvantage of reducing the bar properties at the location of their connection because of the reduction of cross-section area from the machining of the thread.
This method was then improved into making the thread conical rather than parallel, as illustrated on FIG. 2. By taking advantage of the mechanical lock effect of a cone-to-cone contact, the conical-threaded bar connector can reach a performance close to that of the bar itself, and is at present the most widely used system on the market.
However, although providing a tensile strength close to that of the bar, the conical thread connector fails to maintain the ductility of the bar, because of the brittle nature of its failure mode under tensile load.
In order to remedy this, it has been imagined to enlarge the bar end prior to making a thread on it, thereby avoiding to reduce the cross section area of the bars at the location of their connection.
Such an enlargement is best done by forging, but unfortunately lacked the industrial means to produce it effectively and economically. Indeed, forging machines have so far been developed for the mechanical industry, and not for the construction industry. Materials used in the mechanical industry are of regular and accurate dimensions, and of homogeneous properties from one batch to another. By contrast, manufacturers of deformed bars for the reinforcement of concrete are not bound to any dimensional accuracy other than a weight tolerance of up to 5% on the tonnage produced. Worse, the regulations and habits regarding the steel grade used, the shape and dimensions of their ribs and surface vary greatly from one country to another. All of this made the use of an industrially-designed forging press all but impossible.